The present invention relates to fill strategies in the optical kerf, and more particularly, to a fill pattern in the optical kerf which can be used at all levels where planarization is required and which allows the measurement and alignment marks in the optical kerf to be distinguished from the fill background.
The use of fill is known to level loads for etching, chemical mechanical polishing (CMP) or other technological processes used in the formation of integrated circuits. However, in the optical kerf, a CMP process for planarization is a multi-step, multi-process sequence which is quite complicated. The optical kerf is a strip between chips or a frame around chips on a wafer. Measurement and alignment marks which contain the information necessary for photolithography and other processes carried out in chip manufacture are contained in the kerf. The marks need to be read with a high degree of accuracy and are very sensitive to process variations. As such, the marks should be disturbed as little as possible during the chip manufacturing process. Because of the sensitivity of the marks, the optical kerf usually either does not contain fill, or is filled with solid fill so the marks are not disturbed. In these ways, a solid background is provided for the marks so they may be read accurately.
It should be noted that the marks are provided to correct the relative position of the wafer to be exposed and the mask carrying the information for the exposure. The alignment marks on the wafer are read by scanning with an illuminated slit. Scanning occurs on edges of the mark and is directional. Light reflected or scattered by the alignment mark is gathered as the alignment signal. The relative position of the alignment tool and the mark is chosen to generate a maximum alignment signal. The scattering of the light is usually much stronger on an edge of the mark parallel to the path of the alignment tool. Any fill pattern implemented into the optical kerf should be chosen in such a way that it does not disturb the alignment signal by a significant amount. The field imaging alignment technique currently used relies on the contrast (resolution) of the mark to its background. A fill pattern which is below the resolution limit of the alignment tools appears to be gray and is not detected.
U.S. Pat. No. 5,766,260 to Jang et al. and U.S. Pat. No. 5,681,423 to Sandhu describe various methods of forming alignment marks and polishing techniques the kerf area. However, techniques are quite complicated. In order for full planarization by CMP to become feasible, the different areas of the wafer should contain similar area factors or pattern densities. This means that the pattern density of the product array, which includes the chip and comprises most of the wafer, should be matched by the pattern density of the optical kerf. In many products, the pattern density of the product array is between 25% and 50% solid area. Therefore, a goal for the kerf pattern density should be at most 50% solid area. Thus, the known methods of not providing fill in the optical kerf or filling the optical kerf solid with fill are insufficient.
Additionally, it is not possible to provide any additional empty space between the optical kerf and the product array to match the area densities. To achieve the required densities of the chips on the wafer, the space between chip areas needs to be kept as small as possible. However, measures should be taken in order to support polishing. Otherwise, non-uniformities extend from the optical kerf into the chip area. These non-uniformities may result in a degradation of performance in the chip.
Thus, there is a need for a fill strategy which can support full planarization by CMP while keeping the optical kerf at its present size. Additionally, the fill strategy should provide minimal interference with the measurement and alignment marks present in the optical kerf.
A fill pattern in the optical kerf region is provided. The optical kerf is partially filled with fill in a pattern which has little or no impact on the measurement and alignment marks provided in the optical kerf.
In one embodiment of the invention, the fill is formed as an alternating pattern of lines of fill and unfilled space. The lines may have a thickness which is below the resolution limit of an alignment tool used to read the marks.
In another embodiment of the present invention, the fill is provided in the optical kerf in a pattern that can be resolved by the alignment tool. However, the fill is present at such a low level that it does not contribute a significant amount to the alignment signal.
In a further embodiment of the present invention, the fill is incorporated in the optical kerf in areas outside a measurement region where the marks are scanned by the alignment tool.